In the field of what is termed multimedia, including games machines, data encoding is often used to compress audio data, video data and the like in order to record the audio data and the like efficiently on a memory cassette, CD-ROM or the like. Encoding methods such as APCM, DPCM, and ADPCM have long been known as such data encoding methods. These encoding methods basically involve the calculation of data differentials and allow compression and decompression using comparatively simple algorithms.
Further, recent years have seen the adoption of data encoding methods such as the JPEG (Joint Photographic Coding Expert Group) and the MPEG (Moving Picture Expert Group) methods. Data encoding using JPEG, MPEG, and other such methods involve DCT (discrete cosine transformation) processing, quantization processing and the like, which are orthogonal function transformations, and allow comparatively efficient compression and decompression.
Other data encoding methods include those disclosed in Laid-open Japanese Patent Application S. 58-171094 and Laid-open Japanese Patent Application S. 59-35040. The data encoding method disclosed in Laid-open Japanese Patent Application S. 58-171094 is one in which, after the higher order development formula has been developed, data is compressed with reference to the development formula correlation on the time axis. Further, the data encoding method disclosed in Laid-open Japanese Patent Application S. 59-171094 is one in which data compression is carried out by expressing waveform data using two types of data, namely envelope information and the waveform pitch.
However, the following problems have arisen in the various data encoding methods mentioned above, and elsewhere.
The DPCM, DPCM and ADPCM data encoding methods have a problem in that, although they allow the original data to be compressed using a comparatively simple algorithm, compression and decompression are accompanied by substantial sound quality deterioration. For example, in cases such as when there is a substantial amount of variation in the original data, there are instances where differentials of the original data exceed the predetermined data length. As a result, errors occur between the data after decoding and the original data, and these errors become quantization noise and are a cause of sound quality deterioration and picture quality deterioration.
Further, JPEG, MPEG, and other such data encoding methods have the advantage that they can achieve relatively high compression ratios. However, JPEG, MPEG, and other such data encoding methods require complicated algorithms such as DCT and the processing is therefore complicated and a long time is required for processing.
Moreover, the data encoding method disclosed in Laid-open Japanese Patent Application S. 58-171094 requires the calculation of a higher order development formula for the original data, and the encoding and decoding algorithms become complicated. In the data encoding method disclosed in Laid-open Japanese Patent Application S. 59-35040, there is liable to be a substantial error between the data after decoding and the original data because the original data is expressed using only the envelope and the pitch.
The present invention has taken the above situation into account, and the object of the invention is to carry out compression and extension with little encoding or decoding error using simple algorithms in a data encoding device, data decoding device, data encoding method, and data decoding method.